A general understanding can be obtained of the construction of electrically heated backlites by reading my U.S. Pat. No. 4,137,447 which issued Jan. 30, 1979. These backlites are particularly useful as a backlite in a motor vehicle. Under fogging or icing conditions, current is run through the backlite to defog or deice the same thereby providing a clear window in the rear of a vehicle.
No search was conducted on the subject matter of this specification in the U.S. Patent Office or in any other search facility. I am unaware of any prior art more relevant to the subject matter of this specification than that which will be set forth hereinbelow.
Also known at the time of the invention described in this specification is an electrically heated backlite 10, such as shown in FIGS. 1 and 2 of the drawings. Such a backlite 10 is used in vehicles manufactured by the Ford Motor Company. In particular, the backlite is formed from a base which is a glass sheet 12. The glass sheet has an opaque, nonconductive coating 14 which in the embodiment shown in FIG. 1 extends around the entire perimeter of the glass sheet 12. The purpose of this opaque, nonconductive coating is to provide a sight shield for lending more uniform characteristics to the appearance of the tempered glass sheet when viewed from the opposite side of the vehicle. The opaque, nonconductive coating is used to block out the color developed by a layer of silver ceramic material which is used to form the terminal areas for the electrically heated grid lines on the backlite 10. The opaque coating also shields and protects the adhesive system used to mount the backlite in the vehicle from direct sunlight. The color of the opaque, nonconductive coating can be selected to give a better color coordination with the exterior color of the automotive vehicle in which the backlite is to be installed.
In a single operation, a plurality of thin lines 16--16 and larger terminal areas 18--18 are printed on the glass sheet 12 using a silver ceramic material. The thin lines 16--16 are printed on the surface of the glass sheet 12 and the terminal areas 18--18 are printed on the opaque, nonconductive coating 14. In effect, then, the opaque, nonconductive coating 14 is used as a sight shield to block a view of the enlarged terminal areas 18--18 when one views the backlite 10 from the surface which does not have printed material thereon. A conductor strip 20 is soldered at locations identified by the numerals 22--22 to the enlarged terminal areas 18--18.
While the drawings in this case show only the left hand side of the article being manufactured, it is obvious that the right hand side of the same article is being manufactured in the same manner. In this manner a pair of electrical leads are provided to the glass sheet so that an interconnection may be made by these leads to an electrical circuit (not shown) which provides the power to the thin lines 16--16 which in turn provides heat to the glass sheet in order to deice or defog the same.
Certain difficulties have been encountered in manufacturing such heated backlites. As described above, the conductive silver ceramic for the terminal areas has been applied directly over the opaque, nonconductive ceramic coatings. There is a definite mismatch of the expansion coefficients between these two materials when the backlite is heated, formed and quenched in order to shape and temper the backlite. This results in a possible weak solder bond when the conductor strip is subsequently bonded to the silver ceramic coating. Also, a certain porosity can be developed in the curing and firing of the opaque, nonconductive ceramic coating. The increased porosity of the silver ceramic coating also results in a weakening of the bond between the silver ceramic coating and the opaque, nonconductive coating as well as any solder joint formed between the conductor strip and the terminal areas defined by the silver ceramic material.
It is a principal object of this invention to provide an electrically heated backlite in which there are no difficulties for the terminal areas of the silver ceramic materials and a good, reliable bond can be formed between the conductor strip and terminal areas.